Photoemission involves a process whereby a photon impinging on a solid surface causes a photo-electron to be ejected from that surface. The solid surface which emits the photo-electron in response to the impinging photon is referred to as a photocathode. The photo-electron that is emitted from the photocathode can be collected or amplified for detection by other processes, for instance, secondary emission.
The efficiency of the photoemission in photo-electrons per incident photon, and how it changes according to wavelength are characteristics that are associated with photocathodes.
As is well known, photocathodes are used in imaging type photo-detector devices. An example of such a device is an image intensifier tube. Image intensifier tubes amplify a low intensity or non-visible radiational image of an object into a readily viewable image. Image intensifier tubes find many industrial and military applications including enhancing the night vision of aviators, rendering night vision to persons who suffer from retinitis pigmentosa, more commonly known as night blindness and photographing astronomical bodies.
Shown in FIG. 1 is an example of a prior art GEN II image intensifier tube 10 known as a proximity focused image intensifier tube. Image intensifier tube 10 includes an input window 12 formed from a glass or a fiber optic faceplate. The back of the faceplate has applied to it a photocathode 14. A microchannel plate (MCP) 16, which consists of an array of microscopic channels formed in a thin glass plate and serves as electron multiplier, is spaced from and mounted parallel with the photocathode 1414. The MCP 16 includes an input electrode 24 on the input surface and an output electrode 26 on the output surface. An output window 18 is located on the other side of MCP 16 and is provided with a phosphor screen 20. The output window is also formed from a fiber optics faceplate or glass. The input window 12 and output window 18 are mounted on opposing ends of a vacuum housing 22 with the MCP 16 positioned intermediate therebetween within the vacuum housing. The tube is provided with electrical leads for applying appropriate desired voltages to the photocathode 14, the input electrode 24, the output electrode 26, and the phosphor screen 20.
In operation, incident photons coming from an external object impinges upon the photocathode 14. The photocathode 14 converts the incident photons into photo-electrons. The electrons generated by the photocathode 14 are subsequently emitted into the vacuum housing 22. The electrons emitted by the photocathode 14 are accelerated toward the input surface of the MCP 16 by applying a potential applied across the input electrode 24 of the MCP 16 and the photocathode 14.
When an electron enters one of the channels of the MCP 16 at the input surface, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at the output surface of the MCP 16 and are accelerated toward the phosphor screen 20 to produce an intensified image. Electrons exiting the MCP 16 are accelerated toward the phosphor screen 20 by the potential difference applied between the output electrode 26 of the MCP 16 and the phosphor screen 20. As the exiting electrons impinge upon the phosphor screen 20, many photons are produced per electron. The photons create an intensified output image on the output surface of an optical inverter element 28.
A number of materials have been developed over the years which are useful in forming photocathodes for a wide variety applications. Photocathodes used for near UV and visible light are typically made from compounds of alkali metals, usually cesium (Cs), potassium (K), or sodium (Na), with antimony (Sb). These compounds are such that they must be prepared in ultrahigh vacuums.
GEN II image intensifier tubes are well known in the art and use the alkali antimonide, positive affinity, photocathode described immediately above. As described earlier, the photocathode consists of a glass faceplate wherein one surface of the faceplate has deposited thereon a photocathode layer of photoemissive material.
The photocathode layers are deposited onto faceplates using well known chemical vapor deposition techniques. Briefly, chemical vapor deposition (CVD) involves a technique whereby a thin film of either a conductive or insulative material is formed on a substrate by supplying energy for a gas phase reaction. The energy may be supplied by heat, plasma excitation, or optical excitation.
For an example of a method for manufacturing a photocathode used in image intensifier device, see U.S. Pat. No. 4,999,211 entitled APPARATUS AND METHOD FOR MAKING A PHOTOCATHODE issued to Daniel D. Duggan on Mar. 12, 1991 and assigned to the assignee herein, ITT Corporation.
Multi-alkali type photocathodes used in GEN II image intensifier tubes are typically made by a CVD process which involves evaporating for instance, antimony (Sb) onto the surface of a faceplate. An example of such a technique is known as a remote-process system. This technique consists of a "process can", which is usually manufactured from stainless steel. The process can contains the elements that react to form the photocathode layer on the surface of the face plate. The alkali elements are evaporated into the can along with Sb to form the photocathode layer.
The Sb is introduced into the can using an evaporator which consists of a bead of Sb wetted to a wire. Current is passed through the wire, causing the Sb to melt and evaporate onto the faceplate.
This method suffers from several drawbacks. One drawback involves crust formation on the bead of photo-sensitive material. This occurs when the alkali vapor combines with the bead. In order for pure bead material to be available for evaporation onto the faceplate, the crust formed on the bead must be evaporated off first which impacts negatively on the photocathode sensitivity.
Another drawback involves embrittlement of the bead which causes the bead to fall off the wire during the evaporation process thereby, preventing completion of the process. This problem is generally caused by bead being exposed to the Alkali vapor during the evaporation process. Consequently, completion of the process requires two or more beads assemblies to be mounted in the can to provide a backup in case this occurs. This solution is quite costly as each bead and wire assembly is relatively expensive to manufacture.
Therefore, there exists a need in the art of for a method of depositing a photosensitive material onto a faceplate during the manufacturing of a photocathode that avoids the problems of prior art techniques.
It is therefore, a primary objective of the present invention to provide an evaporator that protects the photosensitive material provided thereby from the detrimental effects of the alkali vapor during photocathode formation.